The detection of greenhouse gas emissions has become an important part of ensuring the efficient operation of various systems, and compliance with environmental regulations. For example, remote leak detection from an aircraft or spacecraft platform is essential for efficiently monitoring manufacturing zones, agricultural areas, pipeline systems, drilling operations, and the like. In addition to simple leak detection, it is desirable to provide information regarding the magnitude of a detected leak, and the precise location of the leak. Also, it is desirable to provide such information quickly and conveniently.
One way of obtaining information regarding the amount of atmospheric trace gases is to sense the spectral absorption of reflected sunlight. In particular, the amount of absorption of light at wavelengths corresponding to the spectral lines of the gas of interest can be detected and measured. In general, the higher the absorption of light at such wavelengths, the higher the concentration of the associated gas in the portion of the atmosphere from which the sampled light was collected. Similarly, the absorption of thermal emissions by atmospheric trace gases can be measured to obtain information regarding the amount of such gases. Various spectrometers have been developed for enabling such measurements. For example, Fourier transform spectrometers have been developed that are capable of high spectral resolution. However, such instruments are relatively large and complex. Other instruments for sensing light within a narrow range of wavelengths include devices utilizing optical cavities, such as Fabry-Perot interferometers and multiple cavity filters formed from thin films. However, the sensitivity and signal to noise ratio of such devices has been limited.
One approach to providing a filter having characteristics precisely correlated to the gas being sensed is to provide a cell containing a sample of the gas of interest. By comparing the difference between the light passed through the gas-containing cell to a detector, and light received at a detector that has not been passed through the cell, information regarding the presence of that gas in the atmosphere can be obtained. Although systems using samples of the gas being sensed are capable of providing filter characteristics that are correlated to that gas, they are difficult to implement.
Another approach is to known as a Differential Absorption Lidar (DIAL). In a DIAL system, on line and off line pulses of light are directed towards an area of interest. The on line light has a wavelength that coincides with an absorption line of a gas of interest. The off line wavelength is selected so that it is substantially less affected or unaffected by the gas of interest. By comparing an intensity of light of the first wavelength that has been reflected from the area of interest to the intensity of light of the second wavelength that has been reflected from the area of interest, an estimated amount of the gas of interest that the light has passed through can be determined. In previous DIAL systems, cavity seeding and locking has been used to control laser wavelength and linewidth. However, such systems do not achieve desired levels of laser beam combining and energy profile matching. In addition, previous implementations of DIAL systems have been expensive and complex to implement. Furthermore, previous DIAL systems do not accomplish both gas sensing and 3D topographical imaging simultaneously. Accordingly, previous implementations of these systems have required multiple passes over the area of interest.
In some previous instruments, a 3D imaging system is used in combination with a separate methane sensing system. As another example, a system performs data fusion with respect to data from multiple image sensors and data from a differential absorption LIDAR carried by an aircraft. The method of acquiring data using such a system includes the steps of: (a) turning ON a DIAL sensor to detect a target of interest during a first flight pass over a region of interest (ROI), wherein the target of interest is a gas or oil pipeline leak; (b) detecting the target of interest using the DIAL sensor; and (c) storing location of the detected target in a look up table (LUT). The method also includes the steps of: (d) during a second flight pass over the ROI, triggering another sensor to turn ON at or about the location stored in the LUT; and (e) confirming presence of the target of interest using both ON-sensors. If necessary, a third flight pass over the ROI is conducted and yet another sensor is triggered to turn ON at or about the location stored in the LUT. Presence of the target of interest is confirmed using all three ON-sensors. Accordingly, such systems require multiple passes over an area of interest.